I noticed from trying out the game something very interesting. Apparently, the Brawler, Swordfighter, and Gunner based characters all operate under Pokémon Starter mechanics in that they are strong against one class, but weak against the other.
This stemmed when I tried Mega Man in the demo initially, who has such brutal matchup issues with the likes of Mario, Link, Pikachu, and Villager. Eventually, I tried out the game itself, and noticed that characters like Mario, Link, and Mega Man have some very interesting matchups. Let me put it this way:
Brawlers like to be in your face, and tend to home in on enemies from a distance. Gunners are at a distinct disadvantage, though Swordfighters generally have disjointed hitboxes that can keep Brawlers at bay.
Swordfighters are more or less a tactician based class of fighter. They can keep other characters from getting too close, but struggle against characters who love to camp (Gunners).
Gunners fight from a distance and are primarily campers. They can limit the options of a Swordfighter, though Brawlers can punch right through their bullets.
What's interesting is that according to a leaked list of English translated custom moves (and what each custom move does), the three Mii Classes were the very first characters created (yes, even before the Original 12 minus Jigglypuff), and were likely created first as a means of balancing out the three basic fighter functions of Brawler, Swordfighter, and Gunner.
But wait! Link is a swordfighter, so why does Mega Man have such a poor matchup there? Well, the simple answer is that, typically speaking, not all Brawlers, Swordfighters, and Gunners operate exactly the same from one another, and since Link and Toon Link are both essentially utility belts, they are able to adequately combat Mega Man players. However, certain characters that can be classified as "Gunners" (Duck Hunt, Bowser Jr.) appear to give just about everybody trouble--especially characters like Little Mac and Sonic (though Sonic can handle Duck Hunt a lot better).
In other words, Mega Man can easily prove broken against Marth and Ike players unless custom moves is to become a thing, which if anything, adds to another layer on how useful a character can be. For this reason, how high or low a character may rank on the tier list may not be determined just strictly by their matchups, but in how useful their custom moves can be. Combo potential and easy kills also serve as a huge indicator in where certain characters can rank in the tier list.
Just to use him as a sole example, Shulk was, shortly after having seen the movie and listening to his game's soundtrack, my most anticipated newcomer for this game. I tried him out, and even used his custom moves. His custom moves are rather dreadful, and just about any Gunner-type characters can beat him up easily (though very few Brawlers can get close to him). Custom moves are not very helpful at all, save for some tactical use for his custom neutrals, and Shulk has very low combo potential. He also lacks any sort of cheap exploits either (like Dedede's Gordo Toss).
Truth be told, I'm not horribly impressed with Shulk or Mega Man, though I am very doubtful either of them will be low tier when the game comes out. Mega Man in particular is better than most Gunner classes as well as the most generic Swordfighters. Whereas Shulk seems to be a hard counter for Sonic and Little Mac--that is, when used properly. How each character may rank in relation to one another is dependent on this triangular matchup charts, how effectively each particular character plays in relation to where they're ranked in the matchup charts, and how they compare to other fighters within the same Class in general. I have no doubt in my mind that Link is going to be the best Swordfighter in Smash 4 based on the great amount of utility he has going for him. However, if I were to make a guess on who I think the worst swordfighter in the game is going to be, sadly, Shulk appears to be it.
But I could still be wrong, so who knows? Needless to say, figuring out the Tier List for this installment is not impossible, but it will be very difficult. The game balances itself rather well, and so far, nobody that I could detect is completely worthless as a fighter. But if it's any consolation, I may have found a basis to go by. Just a reminder, here's to keep in mind:
1. Brawler > Gunner > Swordfighter > Brawler (again)
2. How effective each character is in relation to their respective class
3. The Combo and K.O. potential of each character
4. The full extent of each of their custom moves
Brawler: Peach, Bowser, Yoshi, Wario, Donkey Kong, Ganondorf, Kirby, King Dedede, Little Mac, Charizard, Jigglypuff, Captain Falcon, Olimar, Wii Fit Trainer, Pac-Man, Sonic
Brawler/Swordfighter: Greninja
Swordfighter: Marth, Ike, Meta Knight, Lucina, Shulk
Swordfighter/Gunner: Link, Pit, Toon Link, Robin, Dark Pit
Gunner: Bowser Jr., Zelda, Samus, Zero Suit Samus, Palutena, Pikachu, Duck Hunt, Ness, Mega Man
Gunner/Brawler: Mario, Luigi, Rosalina, Mr. Game & Watch, Diddy Kong, Sheik, Fox, Falco, Lucario, R.O.B., Villager, Dr. Mario